The disclosure of Japanese Patent Application No. HEI 10-76727 filed on Mar. 9, 1998 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The present invention relates to a technology for reducing the carbon monoxide concentration in a hydrogen-rich gas containing hydrogen and carbon monoxide that is less in concentration than hydrogen.
2. Description of Related Art
In a typical fuel cell system that uses a hydrogen-rich gas as a fuel, a hydrogen-rich gas is produced by a reformer, and then supplied to a fuel cell. The hydrogen-rich gas is produced in the reformer by introducing thereinto methanol as a fuel material and also introducing water, and causing a water vapor reforming reaction of methanol by using a copper-zinc (Cu-Zn) catalyst, that is, a methanol reformation catalyst. Since the water vapor reforming reaction of methanol is an endothermic reaction, it is necessary to supply heat from outside and maintain an optimal temperature of 200-300xc2x0 C. for the reaction.
The water vapor reforming reaction of methanol can normally be expressed by formula (1):
CH3OH+H2Oxe2x86x923H2+CO2xe2x80x83xe2x80x83(1) 
For a more specific description, the reaction expressed by formula (1) can be divided into two reactions expressed by formulas (2) and (3):
CH3OHxe2x86x92CO+2H2xe2x80x83xe2x80x83(2) 
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(3) 
As is apparent from the above formulas, the water vapor reforming reaction of methanol produces carbon monoxide (CO) as a byproduct.
In some cases, in order to eliminate the need for a heat supply from outside, an oxygen-containing oxidative gas (for example, air) is introduced into the reformer so that along with the water vapor reforming reaction of methanol, a partial oxidation reaction as expressed by formula (4), which is an exothermic reaction, is caused.
CH3OH+1/2O2xe2x86x922H2+CO2xe2x80x83xe2x80x83(4) 
In such a case, too, the water vapor reforming reaction of methanol occurs along with the reaction expressed by formula (4), so that carbon monoxide is still produced as a byproduct. Therefore, in any case, the hydrogen-rich gas produced by this type of reformer contains carbon monoxide.
If the hydrogen-rich gas produced by this type of reformer is directly supplied to a fuel cell, carbon monoxide contained in the hydrogen-rich gas is adsorbed to a platinum (Pt) catalyst provided in an electrode in the fuel cell. If the carbon monoxide concentration in the hydrogen-rich gas exceeds a predetermined allowable level, the correspondingly increased amount of carbon monoxide adsorbed to the Pt catalyst reduces the catalytic function thereof to an undesired level, so that a hydrogen decomposing reaction, that is, an anodic reaction in the fuel cell, is impeded and, therefore, the performance of the fuel cell decreases.
The allowable carbon monoxide concentration in the hydrogen-rich gas supplied to a fuel cell, for example, a polymer electrolyte fuel cell, is about several parts per million.
Therefore, in a typical fuel cell system that uses a hydrogen-rich gas as a fuel, a carbon monoxide concentration reducing device is disposed between the reformer and the fuel cell in order to reduce the carbon monoxide concentration in the hydrogen-rich gas.
The carbon monoxide concentration reducing device has a CO-selective oxidation portion whose interior is filled with a selective oxidation catalyst that selectively accelerates the oxidation of carbon monoxide. An oxygen-containing oxidative gas (for example, air) and the hydrogen-rich gas produced by the reformer are mixed, and the mixture thereof is introduced into the CO-selective oxidation portion, in which carbon monoxide in the hydrogen-rich gas is selectively oxidized by oxygen contained in the oxidative gas via the function of the selective oxidation catalyst so as to reduce the carbon monoxide concentration in the hydrogen-rich gas to a level of several ppm.
This type of carbon monoxide concentration reducing device is described in, for example, Japanese Patent Application Laid-open No. HEI 9-30802, which exemplifies several selective oxidation catalysts including a platinum-ruthenium (Pt-Ru) alloy catalyst, a ruthenium (Ru) catalyst, and the like.
However, this carbon monoxide concentration reducing device has the following problems. That is, immediately after the carbon monoxide concentration reducing device starts to be driven at the time of start of the fuel cell system, the internal temperature of the CO-selective oxidation portion is substantially equal to a room temperature (that is, an ambient temperature of the carbon monoxide concentration reducing device). Since the internal temperature of the reformer is relatively quickly raised to 200-300xc2x0 C. as stated above, introduction of a warmed-up hydrogen-rich gas from the reformer into the CO-selective oxidation portion causes gradual increases in the internal temperature of the CO-selective oxidation portion. The temperature range in which an ordinary selective oxidation catalyst becomes able to perform its catalytic function (hereinafter, described as xe2x80x9cbecomes activatedxe2x80x9d) is considerably higher than the ambient temperature of the carbon monoxide concentration reducing device, that is, a normal room temperature (for example, the aforementioned temperature range is 100xc2x0 C. or higher). Therefore, it takes a considerably long time for the internal temperature of the CO-selective oxidation portion to reach the temperature range in which the selective oxidation catalyst becomes activated, after the carbon monoxide concentration reducing device starts to be driven. That is, a certain length of time is needed in some cases before the carbon monoxide concentration reducing device comes to effectively function. As a result, there is a danger that a longer time may be required before the fuel cell, disposed downstream of the carbon monoxide concentration reducing device, begins to effectively function.
Accordingly, it is an object of the present invention to provide a carbon monoxide concentration reducing apparatus which solves the aforementioned problems, that is, raises the internal temperature of a CO-selective oxidation unit carrying a selective oxidation catalyst as quickly as possible when the apparatus is started, and to provide a driving method for the apparatus.
To achieve some of the objects of the invention, a first aspect of the invention provides a carbon monoxide concentration reducing apparatus for reducing a concentration of carbon monoxide contained in a hydrogen-rich gas by using oxygen contained in an oxidative gas, the carbon monoxide concentration reducing apparatus including: a selective oxidizer into which the hydrogen-rich gas and the oxidative gas are introduced; a selective oxidation catalyst provided in the selective oxidizer, the selective oxidation catalyst selectively facilitating oxidation of carbon monoxide; and an oxidation catalyst provided in the selective oxidizer, the oxidation catalyst facilitating oxidation of at least one component of the hydrogen-rich gas. The oxidation catalyst facilitates the oxidation at least at a temperature substantially equal to an ambient temperature of the apparatus.
In this aspect of the invention, the selective oxidizer contains the selective oxidation catalyst and the oxidation catalyst that facilitates the oxidation of at least one component of the hydrogen-rich gas at a temperature at least substantially equal to an ambient temperature of the carbon monoxide concentration reducing apparatus, that is, substantially equal to a normal room temperature. Therefore, even when the internal temperature of the selective oxidizer is substantially equal to a normal room temperature, the selective oxidizer is able to produce heat through the oxidation of at least one component of the hydrogen-rich gas by oxygen in the oxidative gas, thereby raising the internal temperature of the selective oxidizer.
That is, according to the first aspect of the invention, when the carbon monoxide concentration reducing apparatus is started, the internal temperature of the selective oxidizer can be increased to or above a predetermined temperature in a short time period. Therefore, the internal temperature of the selective oxidizer can be raised to a temperature range in which the selective oxidation catalyst becomes activated, as quickly as possible. Consequently, the carbon monoxide concentration reducing apparatus will quickly become able to effectively function. If the carbon monoxide concentration reducing apparatus of the first aspect of the invention is incorporated in a fuel cell system, a fuel cell disposed downstream of the carbon monoxide concentration reducing apparatus will quickly become able to effectively function.
In this aspect, the carbon monoxide concentration reducing apparatus may further have a construction as follows. That is, the hydrogen-rich gas and the oxidative gas are mixed, and then a gas mixture thereof is introduced into the selective oxidizer via at least one inlet. The oxidation catalyst is provided near the at least one inlet of the gas mixture.
By providing the oxidation catalyst near the inlet, the construction of the selective oxidizer can be simplified and, at the same time, the production cost can be reduced.
The carbon monoxide concentration reducing apparatus may also have a construction as follows. That is, the hydrogen-rich gas and at least a portion of the oxidative gas are introduced into the selective oxidizer via different inlets, and the oxidation catalyst is provided near the inlets.
This construction makes it possible to introduce the oxidative gas substantially uniformly into various portions of the selective oxidizer. Therefore, the internal temperature of the selective oxidizer can be raised as quickly and uniformly as possible.
A second aspect of the invention provides a carbon monoxide concentration reducing apparatus for reducing a concentration of carbon monoxide contained in a hydrogen-rich gas by using oxygen contained in an oxidative gas, the carbon monoxide concentration reducing apparatus including: a selective oxidizer into which the hydrogen-rich gas and the oxidative gas are introduced; a selective oxidation catalyst provided in the selective oxidizer, the selective oxidation catalyst selectively facilitating oxidation of carbon monoxide; and a temperature raiser that raises a temperature of the hydrogen-rich gas to or above a predetermined temperature before the hydrogen-rich gas is introduced into the selective oxidizer.
In the second aspect of the invention, when the carbon monoxide concentration reducing apparatus is started, the temperature raiser raises the temperature of the hydrogen-rich gas to be introduced into the selective oxidizer, to or above a predetermined temperature. Therefore, even if the internal temperature of the selective oxidizer is substantially equal to a normal room temperature at the time of start of the carbon monoxide concentration reducing apparatus, the temperature of the hydrogen-rich gas to be introduced is raised to or above the predetermined temperature, so that the internal temperature of the selective oxidizer can be suitably raised.
That is, in the second aspect, also, the internal temperature of the selective oxidizer can be raised to or above the predetermined temperature in a relatively short time period following start of the apparatus. Therefore, the internal temperature of the selective oxidizer can be raised as quickly as possible to a temperature range in which the selective oxidation catalyst becomes activated.
In the second aspect, the carbon monoxide concentration reducing apparatus may further have a construction as follows. That is, an oxidation catalyst that facilitates oxidation of at least one component of the hydrogen-rich gas is provided in the selective oxidizer. At least when the apparatus is started, the oxidative gas containing oxygen is introduced into the temperature raiser.
This construction makes it possible to more quickly raise the temperature of the hydrogen-rich gas to be introduced into the selective oxidizer at the time of start of the carbon monoxide concentration reducing apparatus. Furthermore, the oxidation of the at least one component of the hydrogen-rich gas for the purpose of raising the temperature of the hydrogen-rich gas is performed by an oxidation device that is provided separately from the selective oxidation device, so that the amount of oxygen available in the selective oxidizer can be used solely for a reaction that reduces the carbon monoxide concentration in the hydrogen-rich gas. Therefore, the carbon monoxide concentration reducing capacity of the selective oxidizer is not reduced but can be fully utilized.
Furthermore, the carbon monoxide concentration reducing apparatus may further include a hydrogen-rich gas producer that produces the hydrogen-rich gas from a fuel material, a supplier that supplies the fuel material to the hydrogen-rich gas producer, and a heater that raises an internal temperature of the hydrogen-rich gas producer, wherein when the apparatus is started, an amount of the fuel material supplied by the supplier is increased at least to a predetermined amount.
This construction also makes it possible to more quickly raise the temperature of the hydrogen-rich gas to be introduced into the selective oxidizer at the time of start of the carbon monoxide concentration reducing apparatus. Furthermore, since a component of the hydrogen-rich gas is not oxidized by using an oxidation catalyst, waste of such a component of the hydrogen-rich gas does not occur.
In the first and second aspects of the invention, the oxidation catalyst may be a platinum (Pt) catalyst. The Pt catalyst is capable of effectively facilitating the oxidation of hydrogen (H2) in the hydrogen-rich gas and therefore generation of heat even at temperatures substantially equal to a normal room temperature.
A third aspect of the invention provides a carbon monoxide concentration reducing apparatus for reducing a concentration of carbon monoxide contained in a hydrogen-rich gas by using oxygen contained in an oxidative gas, the carbon monoxide concentration reducing apparatus including a selective oxidizer into which the hydrogen-rich gas and the oxidative gas are introduced, and a selective oxidation catalyst provided in the selective oxidizer, the catalyst selectively facilitating oxidation of carbon monoxide, wherein at least when the apparatus is started, an amount of moisture contained in the hydrogen-rich gas is reduced to or below a predetermined amount.
Normally, the reaction rate of oxidation catalyzed by the oxidation catalyst and the selective oxidation catalyst increases as the amount of moisture contained in the hydrogen-rich gas decreases. Therefore, by reducing the amount of moisture contained in the hydrogen-rich gas as described above, the reaction rate of oxidation can be improved even at temperatures substantially equal to a normal room temperature. Through an improvement in the reaction rate of oxidation catalyzed by the oxidation catalyst, a further reduction in the carbon monoxide concentration in the hydrogen-rich gas can be achieved. Furthermore, a reduction in the moisture content in the hydrogen-rich gas reduces or eliminates condensation of moisture in the selective oxidizer, so that degradation of the function of the selective oxidation catalyst due to condensation can be prevented.
According a further aspect of the invention, there is provided a driving method for a carbon monoxide concentration reducing apparatus, including the steps of: introducing a hydrogen-rich gas containing carbon monoxide, into a reaction chamber containing a selective oxidation catalyst that selectively facilitates oxidation of carbon monoxide; reducing a carbon monoxide concentration in the hydrogen-rich gas by oxidizing carbon monoxide contained in the hydrogen-rich gas by using the selective oxidation catalyst; discharging the hydrogen-rich gas in which the carbon monoxide concentration has been reduced, from the reaction chamber; and raising a temperature of the reaction chamber to or above a predetermined temperature at least when the apparatus is started.
In this driving method, the temperature of the reaction chamber, in which the selective oxidation of carbon monoxide is caused, is raised to or above the predetermined temperature at least when the carbon monoxide concentration reducing apparatus is started.
Therefore, even if the temperature of the reaction chamber is substantially equal to a normal room temperature immediately after start of the apparatus, the temperature of the reaction chamber can be raised to or above the predetermined temperature in a relatively short time period. That is, the temperature in the reaction chamber can be raised as quickly as possible to a temperature range in which the selective oxidation catalyst becomes activated. Consequently, the carbon monoxide concentration reducing apparatus will quickly become able to effectively function.